Black and blue is to this decade what black and red was to the 80s. Forget those daft bossy rhymes - blue and green should never be seen, and so on - because there is no such thing as a good or a bad colour combination. Colours change in different contexts, just as they do in different lights. (As Diana Vreeland famously said, pink is the navy blue of India.) Purple and brown looked fantastic in 1972; 15 years later, the same combination would have had people sniggering behind their pina coladas. You need timing.

You need confidence, too. Wearing blue and black together takes a bit of getting used to. It's a combination that is neither all-out moody, like black and grey, nor straight-up chipper, like blue with white, but betwixt and between, like a shadow on a sunny day. Handy for days that are neither red-letter, nor a washout, but somewhere in the middle.

However, done badly, black and blue looks like a cheap-trying-to-be-cheerful school or checkout uniform. Be aware of this pitfall in daywear - a boxy navy blazer and straight black skirt, for instance, are to be avoided. To give this particular danger a wide berth, you need to add to the mix something out of the workaday: a slinky fabric, a spot of sharp tailoring, foxy shoes, whatever. It is tempting to opt for baby blue rather than bright blue with black, because it seems like a softer combination, but that looks like business wear. There's nothing wrong with that, but it's not fashion, darling.

After dark, mixing electric blue and black can look early 80s. This needn't be a bad thing. It's probably best to avoid teaming a black and blue outfit with enormous clip-on earrings or white tights, but apart from that, the world's your oyster - or prawn cocktail, as you wish.